Viscoelastic short cracks propagation

It is known that energy balance is the correct criterion for large enough cracks, as otherwise the limit (cohesive) strength dominates, and this is the reason for many examples in Nature of “flaw insensitive” design at very small scales. These concepts have so far been proposed only for elastic materials, neglecting the effect of viscoelasticity. This introduces some novelty: for liquid materials, there is never flaw insensitivity as slow crack propagation occurs in principle at any crack size. On the other hand, for materials with a well defined relaxed modulus, flaw sensitivity can be defined with respect to threshold or very fast speed of propagation, respectively. These concepts are introduced by means of a simple Dugdale model, in which Schapery's approximation is used. Qualitative comparison with existing theories based on the de Gennes/Persson-Brener model with dissipated energy is made, and large differences are found.